In the manufacture of fiber from molten material, it has been common practice to use electrically heated bushings made of precious metals including platinum, rhodium, palladium, ruthenium, iridium and alloys thereof. The bushings are electrically heated by their own resistance and are usually box-like, open on the top and comprise an orifice plate containing hundreds or thousands of orifices, with or without nozzles or tips welded or formed thereon, as shown by U.S. Pat. Nos. 4,207,086 and 4,078,413, which disclosures are hereby incorporated by reference.
As the molten material emerges from the orifices or tips, a meniscus or cone of molten material is formed below each orifice or tip from which a fiber is pulled continuously. This is the objective and to achieve good fiberizing efficiency and productivity, it is essential to uniformly remove a lot of heat from the ends of the tips, the cones of molten glass and the fibers close to the tip plate. What happens in the first half inch or so below the ends of the tips is very critical and if not right one or more fibers will break, requiring a costly stoppage of desired fiberization from that bushing and a beading down and restart to achieve the desired fiberization. By desired fiberization is meant that the bushing is operating making the desired fiber for the product being produced. To remove the heat from the meniscus and fiber that must be removed to cool the molten or plastic fiber so that it will have integrity and strength to endure the remainder of the process of making the fiber product, cooling members are located close to the orifices or nozzle tips. These cooling members can be either cooling tubes like shown in U.S. Pat. Nos. 3,628,930, 4,397,665, 5,244,483 and 6,196,029, the disclosures of which are hereby incorporated by reference, or cooling fins, sometimes called fin blades or fin shields, attached to a cooling manifold as are well known in the fiber industry.
Occasionally, and sometimes frequently, a fiber will break beneath the bushing for various reasons that are known. If the quality of the molten material is satisfactory for fiberization, most of the fiber breaks are caused by one or more tips on the bushing plate being hotter, but usually colder, than desired for good fiberization. A cold tip makes a smaller diameter fiber due to a lower flow rate of molten material through the cold tip. This lower flow rate causes the meniscus and initial fiber to undergo a greater attenuation rate and the greater attenuation rate, of the colder material creates more stress in the attenuation zone. The finer fiber in the attenuation zone also results in faster cooling in the attenuation zone that adds to the stress. When the stress reaches a critical level, the fiber breaks. If the tip is too hot, the viscosity becomes too low to resist the surface tension force of the material and the fiber breaks.
When a fiber break occurs, the loose fiber soon causes other fibers to break and soon all, or most, fibers being formed beneath the bushing are broken, a stoppage of desired fiberization. This is called a “breakout” in the industry. After a breakout begins, it is necessary to wait a short time, usually tens of seconds up to a few minutes, for beads of molten glass to form beneath each bushing orifice or tip, and become large enough that they break loose and fall from the bottom of the orifice plate or tip pulling a very coarse fiber, called a primary fiber, onto the floor, into a scrap bin, basement or scrap bin beneath the forming room floor. This is normally called “beading out” in the industry and the “beading out” typically takes about 30-120 seconds. Once beaded out, or as soon as the operator is available, the operator or starting equipment can then restart a strand containing the primary fibers into a chopper or winder and again begin making the desired product.
When the bushing is running good product the fibers are moving away from the bottom of the bushing at a speed of thousands of feet per minute. This downward movement at this speed, of an array of hundreds or thousands of fibers, creates, due to friction between the air surrounding the fibers and the surface of the fibers, a partial vacuum (lower pressure zone) by pulling a stream of air downward. The partial vacuum causes a flow of cooling air from the surroundings into the array close to the orifice plate and tips of the bushing. This flow of inspirated air coming from outside the array of fibers cools the tips, meniscuses and the newly formed fibers somewhat. Additional cooling is accomplished with the cooling members, cooling fins or cooling tubes, mounted beneath the bushing and close to the tips to cool the air tips and glass/fibers. The cooling of the bushing, tips and orifice plate, causes additional electrical power to be applied to the bushing automatically to maintain the set-point temperature.
When the bushing breaks out, this inspirated cooling flow of cooling air stops. At that time several more undesirable things begin. The set point thermocouple begins to heat up because of the loss of cooling air and as it does, the controller decreases the electrical power heating the bushing. As the electrical power is decreased during the beading out and hanging periods, the molten glass through-put decreases by 5-15 percent, slowing the flow of molten glass through the well, orifice, between the forehearth leg above the bushing and the bushing causing the temperature of the molten glass in the well, and thus the molten glass entering the bushing, to drop substantially, about 25-75 degrees F. This colder glass coming into the bushing causes the molten glass exiting the orifices to be colder and thus to have a higher viscosity. The higher viscosity glass has more resistance to attenuation when desired fiberization is restarted, causing higher stress in the fiber at its weakest point, and it frequently breaks. This is why the break rate is normally highest during the first ten minutes or longer after restart of desired fiberization, particularly as the area of the orifice/tip plate of the bushings has increased to accommodate mote orifices/tips. The larger the area of the orifice plate or tip plate, the greater the tendency to have a larger temperature variance across the orifice plate or tip plate or the tips. It normally takes about ten minutes or longer for the molten glass in, and exiting, the bushing to again reach the desired fiberizing temperature.
The above conditions apply to any molten material, but are most costly in the manufacture of so-called “continuous” glass fiber products from molten glass. In the manufacture of continuous glass fibers, melting furnaces are typically used to melt batch, refine the molten glass, and to feed molten glass through one or more forehearths and usually a plurality of bushing legs to the bushings. It is extremely important, to achieve a very low bushing breakout rate, that the molten glass coming to the bushings is fully melted and uniform in temperature and chemistry. Mixing in the molten glass is mainly dependent upon maintaining desired temperature gradients and flow patterns in the melting furnace. There are typically hundreds of thousands of pounds of molten glass, often about 500,000 pounds, in a typical melting furnace system for making continuous glass fibers. With this much molten material, the melting furnace and delivery system has great momentum and inertia, i.e. it is difficult and takes considerable time correct a change in the molten glass reaching the bushings following a furnace upset. A furnace upset is anything that makes a significant change in the way the melting furnace is operating, including a significant change in the throughput of molten glass through the delivery system, including the bushings. In the past it has been noticed that when a plurality of bushings were stopped from making desired fiber product and put into a hanging mode, to permit a chopper that had been pulling strands of fibers from the bushings to be rebuilt, that after a few minutes the conditions inside the melting furnace would change and that the automatic burner controls for the melter were changing conditions of the burners responding to the change(s) in the furnace. This happens on a smaller scale with every bushing breakout. This is necessary, but not desirable. Although improvements in melting furnace control and stability have been made through the decades that large melting furnaces have existed, frequent furnace upsets or disturbances still exist result in lower productivity and higher manufacturing costs.
There is much debate in the industry whether cooling tubes, usually having at least one fin, or cooling fins, attached at one or both ends to a cooling manifold, are best for making glass fibers. Most of the industry using cooling fins, but at least two fiber manufacturers use finned cooling tubes to manufacture a substantial amount of continuous glass fiber products. Cooling tubes are always mounted to run along the length of the bushing while the cooling fins run across the width of the bushing. As the width of the bushing has grown larger and larger, those using fins have begun using two sets of fins, each set reaching almost one-half the width of the tip plate and each set mounted at one end to a cooling manifold. Temperature gradients of up to several hundred degrees F., up to 800 degrees F. can exist between the upper edge of a fin blade furthest away from the cooling manifold and the temperature of the fin blade where it attaches to the cooling manifold whereas with fins on cooling tubes the top of the fin is almost the same temperature across the length of the fin because the top of the fin is very close to the top of the cooling tube carrying cooling water, usually less than about 6 mm and typically less than about 4 mm. The heat transfer through solid fins is governed by the relationship of thermal conductivity of the fin material times the cross sectional area of the fin times the delta temperature in the fin and divided by the distance of the heat transfer in the fin. As bushings became larger, and throughput higher, to produce more fibers per position and more pounds of fiber per hour per position, this relationship results in less uniformity wile the need is more uniformity in cooling by the cooling members and this uniformity becomes more critical to low breakout rates. Also, cooling tubes are more efficient heat removers per unit of surface area than cooling fins alone making cooling tubes with or without fins easier to accommodate beneath the tip plate without interfering with the fiber paths or bead drop.
There has not been much change in cooling tubes over the decades, except for the material used to make the tubes and fins, but there have been many proposed changes in the design and mode of function of the cooling fins. In U.S. Pat. No. 3,867,118, one set of fins mounted on a coolant cooled manifold extended entirely across the width of the bushing and for a substantial distance on beyond each edge of the bushing. In U.S. Pat. No. 4,330,311, much longer cooling fins connected on each end to a coolant manifold were disclosed. U.S. Pat. No. 4,566,890 and patents mentioned therein disclose methods of retarding or removing deposits of oxides that form on cooling fins causing changes in the heat absorption rate. U.S. Pat. No. 4,824,457 discloses the use of hollow cooling fins in which a coolant fluid is circulated across the fin to the end of the fin and then returned to the coolant manifold in a lower passage the same fin.